According to The Hill, Democrats see Obama's email network as an excellent fundraising source, since Obama's campaign raised around $500 million through email and online donations in 2012.

Clinton supporters said the likely Democratic nominee's fundraising network is a strong one, too, but access to Obama's list will help.

"The best email list is the one you build yourself," one Clinton ally said, reports The Hill. "But it doesn't hurt. The more resources they can piece together the better."

The Democratic National Committee gained access to the list in 2015 when it formed a committee to help the Obama campaign pay off its nearly $2.4 million debt. Sources told The Hill the Obama campaign owns the list and is renting it to the Democratic committee.

A Democratic National Committee official said the email list contains "millions of engaged Democrats across the country," reports The Hill.

One former Obama aide noted that the list might not be as valuable as it once was, because email addresses in it could be outdated. "It's four years old," the former aide said, according to The Hill.

Sending out emails through his list is no guarantee that Obama's supporters will be swayed to vote for Clinton, according to one source in the Hill report. "President Obama can't just hand the movement over to her."

In 2014, The Wall Street Journal reported the nonprofit group Organizing for Action rented Obama's list for $1.2 million per year. Former Obama campaign manager Jim Messina said the list had more than 30 million subscribers, according to the Journal report.